The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to MR imaging of the lungs.
Emphysema is characterized by a breakdown in the alveolar walls of the lung. The diagnosis of emphysema is typically made using the whole lung pulmonary function tests and is characterized by increases in airway obstruction and diffusion abnormalities. To quantify regional emphysematous changes in the lung, high resolution x-ray CT images typically are used to measure the fraction of the lung with Hounsfield Units (HU) below a given threshold. The voxels below this threshold contain mostly air and thus are likely regions of the disease. Breath-hold high speed x-ray CT enables a direct visualization of the lung tissue, but it does not provide physiological information that differentiates between disease induced conditions that arise from emphysema, asthma and other lung diseases. Repeated x-ray CT examination to follow the progression of a lung disease is also limited due to the exposure to ionizing radiation.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
A number of MR imaging techniques have been discovered which provide physiological information about the lungs that enable early diagnosis of diseases and the evaluation of disease progression. These methods have been limited by the low signal-to-noise ratio (SNR) signal from the highly aerated lung tissue. More recently, however, interest in MR imaging of the lungs has increased due to the use of noble gases. A noble gas such as Xenon-129 or Helium-3 is inhaled into the lungs prior to the MRI scan to increase SNR of the NMR signals received at the appropriate Larmor frequency. The noble gas is thermally or equilibrium polarized and is preferably hyperpolarized to produce a strong NMR signal when excited at its Larmor frequency. Such imaging methods are disclosed, for example, in U.S. Pat. Nos. 5,789,921; 6,241,966; 6,338,836; 6,370,415; 6,589,506 and in published U.S. Pat. Appln. Nos. 2001/0031242; 2002/0043267; 2002/0198449; 2003/0023162 and 2004/0260173.
Diffusion-weighted imaging (DWI) is a powerful MRI technique for probing microscopic tissue structure. In DWI, a pulse sequence is employed which contains a magnetic field gradient known as a diffusion gradient that sensitizes the MR signal to spin motion. In a DWI pulse sequence the detected MR signal intensity decreases with the speed of spin diffusion in a given volume. The first moment of this diffusion gradient, also known as the “b-value” determines the speed of diffusion to which the image is sensitive. This b-value may be adjusted by either varying the area of the two lobes of the diffusion magnetic field gradient, or by varying the time interval between them. When spin motion in the subject is unrestricted, the MR signal intensity at the center of the echo using a spin-echo diffusion-weighted pulse sequence is related to the b-value as follows:
                    A        =                                            S              ⁡                              (                b                )                                                    S              0                                =                      ⅇ                          -              bD                                                          (        1        )            where the “b-value” b=γ2G2δ2(Δ−δ/3). The parameter γ is the gyromagnetic ratio of the excited spin species and G is the amplitude of the applied diffusion magnetic field gradients. S(b) is the MR signal magnitude with diffusion weighting b, and S0 is the MR signal magnitude with no diffusion weighting (b=0). The parameter D is the diffusion coefficient of the fluid (in mm2/s), which directly reflects the fluid or gas viscosity where there are no structural restrictions to diffusion of the water or gas. Δ is the time interval between the onsets of the two diffusion gradient lobes and δ is the duration of each gradient lobe. The diffusion coefficient D in equation (1) may be calculated, since b is known and the attenuation A can be measured.
                    D        =                              ln            ⁡                          (                              So                                  S                  ⁡                                      (                    b                    )                                                              )                                /          b                                    (        2        )            The diffusion coefficient D when measured in the presence of structure that restricts diffusion is called the “apparent diffusion coefficient” or “ADC”.
Recently, ADC images of the lungs have been produced using DWI methods. One of the difficulties, however, in using such MRI images for diagnostic purposes is the inability to discern the boundary between the highly aerated lung tissues and the respiratory bronchioles which these tissues surround. The measurements that can be made with MR imaging techniques have vastly different meaning depending on whether the measured voxel resides in an air way or tissue. With the ADC measurement, for example, the diffusion of water in blood is measured in tissue, whereas the diffusion of a noble gas is measured in the air spaces of the airway tree. Many lung diagnostic images depend on depiction of the airway tree, both for detection of abnormal morphology and for locating sites of disease. Therefore, there is a need for a procedure that enables a segmented image to be produced which can be registered with diagnostic MR images and which enables the airway tree in the lungs to be delineated in such images.